what is my diagnostic algorithm for acute-onset blindness in a dog with no prior ocular history?

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The diagnostic algorithm for acute-onset blindness in a dog with no prior ocular history proceeds in three sequential tiers: localize the lesion, characterize retinal function, then pursue advanced imaging. Begin with a complete neuro-ophthalmic examination to localize the lesion. Assess menace response, pupillary light reflexes (PLRs), dazzle reflex, and direct fundoscopic examination. Acute vision loss arises from three broad anatomic categories: opaque ocular media (e.g., diffuse corneal edema, cataracts), retinal dysfunction, or optic pathway disease (optic nerve, chiasm, tracts, or visual cortex).9 True ocular media disease is largely ruled out by anterior segment evaluation and tonometry at this step.2 Electroretinography (ERG) and chromatic PLR testing are the pivotal second-tier tests that distinguish retinal from post-retinal disease. Dogs with photoreceptor-layer disease — including sudden acquired retinal degeneration syndrome (SARDS) and immune-mediated retinitis (IMR) — have an absent or markedly attenuated PLR to red light (630 nm, which stimulates photoreceptors) but a preserved PLR to blue light (480 nm, which stimulates retinal ganglion cells).23 SARDS produces an extinguished ERG tracing, whereas early IMR may produce a normal, high, or low ERG.2 Ivermectin toxicosis produces markedly attenuated outer retinal responses on both ERG and chromatic PLR and should be considered even without a known exposure history, including in MDR1-negative dogs.3 When ERG and PLR are both normal despite complete visual deficits, the lesion is post-retinal — as demonstrated in a Welsh Corgi with optic chiasm compression from hydrocephalus, where ERG and PLR were normal bilaterally.4 When retinal function is intact, MRI with contrast is the definitive third-tier test to characterize the optic pathway and intracranial disease. Retrobulbar and intracranial differentials include optic neuritis, retrobulbar abscess or neoplasia, and disease of the optic chiasm, optic tract